Method for operating a combined cycle power plant

ABSTRACT

A method for operating a combined cycle power plant, according to which shortly before the planned start-up of a parked load, the steam turbine is lowered to a very low output, the gas turbine is then operated in parked load, and next the steam turbine is powered up to a parked output. The GT operating power can be the rated power of the gas turbine. The ST operating power can be the rated power of the steam turbine.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2014/064182 filed Jul. 3, 2014, and claims the benefitthereof. The International Application claims the benefit of EuropeanApplication No. EP13177932 filed Jul. 25, 2013. All of the applicationsare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for operating a combined cycle powerplant, wherein the gas turbine is operated at a GT operating power andthe steam turbine is operated at an ST operating power, wherein thepower of the steam turbine is reduced to an ST part power, wherein theST part power is lower than the ST operating power.

BACKGROUND OF INVENTION

Combined cycle power plants are used to generate electrical energy forcommunal energy supply. In general, a combined cycle power plantsupplies electrical energy to a supply grid, the energy requirementbeing dependent on the time profile. This means that the energyrequirement is not constant over the course of a day. The electricalsupply grid is supplied with electrical energy by multiple power plants.Thus, use is made for example of conventional power plants and powerplants which convert renewable energies into electrical energy.Feeding-in of the renewable energies is subject to fluctuations, whichleads to increasing demands on the conventional power plants. This meansthat conventional power plants must be operated longer and lower in whatare termed part loads or parked loads. In combined cycle power plants,such low part loads are associated—depending on the configuration of thegas turbine—with reduced gas turbine outlet temperatures.

As a result, the steam turbine inlet temperature also drops. Thus, assoon as the plant is operated in part load, the steam inlet temperatureis reduced. However, this means that the hot components of the steamturbine are exposed to cold steam, which leads to thermal stresses.

If the parked load is then abandoned again, the steam temperatures riseagain, which once again leads to thermal stresses. In order to preventthese thermal stresses, it is possible not to run the gas turbine so farin part load in order that the steam temperature does not drop so much.It is also possible to slowly reduce the steam temperature by sprayingprior to the actual load reduction. Then, the load change takes place ata lower—but therefore more constant—temperature. After increasing theload, the steam temperature is once again raised slowly to the ratedtemperature.

Another possibility for preventing thermal stresses consists in runningthe steam turbine down prior to reducing the gas turbine power. Then,the components of the steam turbine will cool down with very low thermalstresses. Once the components have cooled sufficiently, the steamturbine could be started up again at a reduced gas turbine power andthus at a low steam inlet temperature. This would lead to very lowservice life consumption.

SUMMARY OF INVENTION

The invention is based on an object of indicating another possibilityfor reducing thermal stresses.

This object is achieved by a method for operating a combined cycle powerplant, wherein the gas turbine is operated at a GT operating power andthe steam turbine is operated at an ST operating power, wherein thepower of the steam turbine is reduced to an ST part power, wherein theST part power is lower than the ST operating power, wherein the power ofthe gas turbine is then reduced to a GT parked power, wherein the GTparked power is lower than the GT operating power.

Once the GT parked power of the gas turbine has been reached, the powerof the steam turbine is increased to an ST parked power, wherein the STparked power is 20% to 60% of the ST operating power.

The invention thus proposes indicating an operating method wherein thesteam turbine is involved in the parked load. Thus, for the purposes ofgrid stability, the steam turbine rotor keeps as much rotating mass aspossible on the grid.

The ST power of the steam turbine is reduced to a very low power shortlybefore the planned commencement of the parked load. The gas turbine isthen operated in parked load. On account of the markedly lower heattransfers between steam and steam turbine components in part load,service life consumption is significantly lower as a consequence oflowering the steam temperature. In that context, the steam turbine coolsdown slowly.

Advantageous developments are specified in the dependent claims. Thus,in a first advantageous development, the ST part power is set at 5% to40%, 5% to 30%, 5% to 20%, or 5% to 10% of the ST operating power.

In another advantageous refinement, the GT parked power is 20% to 60% ofthe gas turbine operating power.

It is thus proposed to once again take up load after the steam turbinehas cooled down slowly at a reduced gas turbine power and thus a lowsteam inlet temperature.

In one alternative embodiment, the steam turbine could be held in thislow part load until the end of the parked load.

The invention thus proposes reducing the power of the steam turbine toan ST part power. The ST part power is lower than the ST operatingpower. Reducing to the ST part power is effected by closing a steaminlet valve. In that context, the steam inlet valve is controlled suchthat almost no fresh steam flows through the steam turbine. In thatcontext, a bypass station is formed such that there results a fluidicconnection between the steam inlet and the condenser. Thus, downstreamof the steam generator, steam is not fed to the steam turbine butdirectly to the condenser, which has a disadvantageous effect on theefficiency. The steam turbine then cools down. After this, the power ofthe gas turbine is reduced to a GT parked power. This affects the steaminlet temperature. That means that the steam inlet temperature drops.After a certain time, the steam inlet valve is then once again openedand the fluidic connection between the steam inlet and the condenser isinterrupted. Thus, all of the steam generated in the steam generator isthen fed through the steam turbine.

In one advantageous development, the steam turbine comprises ahigh-pressure, intermediate-pressure and low-pressure turbine section,wherein—the high-pressure turbine section,—the high-pressure turbinesection and the intermediate-pressure turbine section,—theintermediate-pressure turbine section,—the intermediate-pressure turbinesection and the low-pressure turbine section or—the low-pressure turbinesection is/are not charged with steam.

Thus, ideally, one pressure stage is completely closed. In theswitched-off turbine section, the service life consumption is even lowersince here the components cool down naturally. Advantageously, thepressure in the steam turbine or in the remaining operational turbinesections is reduced as much as possible, which is made possible bydrains, evacuation lines, start-up lines or also process steam lines.

Thus, the significant reduction in the pressure in the steam turbinereduces the heat transfer and significantly reduces the service lifeconsumption during part load.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to anexemplary embodiment. In the drawing:

FIG. 1 shows a schematic representation of a combined cycle power plant.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a schematic representation of a combined cycle power plant.In essence, a combined cycle power plant 1 comprises a gas turbine 2that can be operated with fossil fuels. This gas turbine 2 comprises acompressor part 3 in which air is heated and compressed, a combustionchamber 4 in which the air from the compressor part 3 is mixed with fueland ignited, and a turbine part 5 in which—in various stages consistingof guide vanes and rotor blades that are not shown—the hot exhaust gasesturn a rotor. A shaft 6 transfers this rotation to a generator 7. Thegenerator 7 then supplies a supply grid with electrical energy (notshown).

The hot exhaust gases from the gas turbine 2 are fed into a steamgenerator 8. In this steam generator 8, fresh steam is generated bymeans of a line 9 and is then fed via a steam turbine fresh steam line10 into a high-pressure turbine section 11. An HP valve 12 is arrangedin the steam turbine fresh steam line 10. The steam leaving the HPturbine section 11 is conveyed to an intermediate superheater 13. Thistakes place via the cold intermediate superheater line 14. Once thesteam has been heated in the intermediate superheater 13, the hotintermediate superheater line 15 supplies steam to anintermediate-pressure turbine section 16. From the intermediate-pressureturbine section 16, the steam flows via an overflow line 17 into twolow-pressure turbine sections 18. After the low-pressure turbine section18, the cold, expanded steam flows into a condenser 19 where itcondenses to water which is conveyed, via a pump 20, back into the freshsteam generator 8 via the fresh steam line 9.

The steam turbine fresh steam line 10 is fluidically directly connectedto the condenser 19 via a redirection station 21. An overflow valve 22is arranged in the overflow line 21. An electric generator 23 isconnected, in a torque-transmitting manner via a common shaft 24, to thehigh-pressure turbine section 11, the intermediate-pressure turbinesection 16 and the low-pressure turbine section 18. The HP turbinesection 11, the IP turbine section 16 and the LP turbine sections 18form the steam turbine 25.

In one alternative embodiment, the combined cycle power plant comprisesa redirection system. This redirection system comprises a high-pressureredirection station 22 and a high-pressure redirection valve 21 arrangedin the high-pressure redirection station 22, wherein the high-pressureredirection station 22 establishes a fluidic connection between thesteam turbine fresh steam line 10 and the cold intermediate superheaterline 14. Furthermore, the redirection system comprises anintermediate-pressure redirection station 22 a and anintermediate-pressure redirection valve 21 a arranged in theintermediate-pressure redirection station 22 a, wherein theintermediate-pressure redirection station 22 a establishes a fluidicconnection between the hot intermediate superheater line 15 and thecondenser 19.

Thus, steam can flow from the steam turbine fresh steam line 10 to thecondenser 19, via the redirection system comprising the high-pressureredirection station 22 and the intermediate-pressure redirection station22 a.

Furthermore, the combined cycle power plant comprises anintermediate-pressure valve 12 a arranged in the hot intermediatesuperheater line 15.

Now, according to the invention, the combined cycle power plant isoperated as follows. First, the gas turbine 2 is operated at a gasturbine operating power. Equally, the steam turbine 25 is operated at anST operating power. The power of the steam turbine 25 is reduced to anST part power, wherein the ST part power is lower than the ST operatingpower. Then, in this context, the ST part power is 5% to 40%, 5% to 30%,5% to 20%, or 5% to 10% of the ST operating power.

This is achieved by nearly closing the HP valve 12 and theintermediate-pressure valve 12 a, such that almost no steam flowsthrough the steam turbine 25. Thus, the components in the steam turbine25 cool down. After a certain time, the power of the gas turbine 2 isthen reduced to a GT parked power, wherein the GT parked power is lowerthan the GT operating power. In this case, the GT parked power is 20% to60% of the gas turbine operating power. As a result, the temperature ofthe hot exhaust gas from the gas turbine 2 is lower, leading to areduction in the temperature of the fresh steam which is generated inthe steam generator 8 and passes through the steam turbine fresh steamline 10 and the hot intermediate superheater line 15.

Once the HP valve 12 is almost closed, the overflow valve 22 or theredirection system 22, 21; 22 a, 21 a is opened such that the majorityof the steam generated in the steam generator 8 is fed directly into thecondenser 19. However, this is disadvantageous for the overallefficiency of the combined cycle power plant.

Once the GT parked power of the gas turbine 2 has been reached, thepower of the steam turbine 25 is increased to an ST parked power. ThisST parked power is 20% to 60% of the ST operating power. This isachieved by opening the HP valve 12 and the intermediate-pressure valve12 a. The overflow valve 22 in the overflow line 21 is closed again.Thus, the steam—now conveyed in the steam turbine fresh steam line 10and in the hot intermediate superheater line 15 as a consequence of thereduced steam inlet temperature of the steam—can be fed into the HPturbine section 11. As a consequence of the lower fresh steamtemperature, the volumetric flow of fresh steam is also lower.

The power of the steam turbine 25 is reduced by reducing the pressure ofthe steam. It is now possible, once the ST part power and the GT parkedpower have been reached, to operate the steam turbine 25 as follows. Thesteam turbine 25 comprises a high-pressure turbine section 11, anintermediate-pressure turbine section 16 and a low-pressure turbinesection 18, wherein the high-pressure turbine section 11, thehigh-pressure turbine section 11 and the intermediate-pressure turbinesection 16, the intermediate-pressure turbine section 16, theintermediate-pressure turbine section 16 and the low-pressure turbinesection 18 or the low-pressure turbine section 18 is/are not chargedwith steam. The remaining turbine sections remain closed and can cooldown naturally.

The pressure of the steam in the turbine sections not charged with steamis then reduced as far as possible. To that end, drains, evacuationlines, start-up lines or process steam lines are opened.

1. A method for operating a combined cycle power plant, comprising:operating the gas turbine at a GT operating power and operating thesteam turbine at an ST operating power, reducing the power of the steamturbine to an ST part power, wherein the ST part power is lower than theST operating power, reducing the power of the gas turbine to a GT parkedpower, wherein the GT parked power is lower than the GT operating power,wherein once the GT parked power of the gas turbine has been reached,increasing the power of the steam turbine to an ST parked power.
 2. Themethod as claimed in claim 1, wherein the ST part power is 5%-40%,5%-30%, 5%-20%, or 5%-10% of the ST operating power.
 3. The method asclaimed in claim 1, wherein the GT parked power is 20%-60% of the GToperating power.
 4. The method as claimed in claim 1, wherein the STparked power is 20%-60% of the ST operating power.
 5. The method asclaimed in claim 1, wherein the power of the steam turbine is reduced byreducing the pressure of the steam.
 6. The method as claimed in claim 1,wherein the steam turbine comprises a HP, IP and LP turbine section andwherein the HP turbine section, the HP turbine section and the IPturbine section, the IP turbine section, the IP turbine section and theLP turbine section or the LP turbine section is/are not charged withsteam.
 7. The method as claimed in claim 6, wherein the pressure of thesteam in the turbine sections not charged with steam is reduced to belowa limit value.
 8. The method as claimed in claim 1, wherein the drain,evacuation lines, start-up lines or process steam lines are opened.